2,5-dihydroxyterephthalic acid (2,5-dihydroxyterephthalic acid may be also referred to as DHTA hereinafter) is an aromatic dicarboxylic acid having two hydroxyl groups. DHTA is useful as a monomer for manufacturing synthetic resins such as polyesters, polyamides, aramids and the like and as a raw material for synthesizing fluorescent agents, intermediates of medicines and the like. Therefore, the demand for DHTA is growing year by year.
A conventionally known process for introducing carboxyl group(s) into a compound having phenolic hydroxyl group(s) is the Kolbe-Schmitt reaction. Examples of the Kolbe-Schmitt reactions include a chemical reaction which comprises contacting an alkaline metal phenoxide (an alkali metal salt of phenol) with carbon dioxide at a high temperature under a high pressure to carboxylate the ortho position of the alkaline metal phenoxide and neutralizing the resulting compound with an acid to give salicylic acid.
With regard to processes for the preparation of DHTA, Patent document 1 (Japanese Patent Application Laid Open No. Sho 49-11841) discloses a process for preparing DHTA by using the Kolbe-Schmitt reaction. However, in this process, the dicarboxylation reaction of hydroquinone to provide DHTA requires a very high pressure. Further, according to this process, DHTA can only be produced with a low conversion ratio and in a low yield.
As a process for introducing carboxyl group(s) under relatively mild conditions, Patent document 2 (Japanese Patent Application Laid Open No. Hei 11-515031 corresponding to WO 97/17315) discloses a process which comprises contacting a dihydric phenol with carbon dioxide in the presence of an alkali metal carbonate and of an alkali metal formate at a temperature above the melting point of the formate.
According to the process disclosed in Patent document 2, DHTA can be obtained in a relatively high yield, but the process requires a large amount of an expensive alkali metal formate. Further, the process involves generation of carbon monoxide. Therefore, the process is not industrially advantageous in terms of safety. Furthermore, this process has a problem of insufficient stirring because a large amount of the alkaline metal formate becomes solidified when the reaction system is cooled after the completion of the reaction. Additionally, this process has another problem of damaging stirring machines and the like because the contents of the reaction system after the completion of the reaction tend to strongly adhere to the reaction equipment.
In order to improve the process disclosed in Patent document 2, Patent document 3 (Japanese Patent Application Laid Open No. 2011-26232) discloses a process for preparing DHTA which comprises a dicarboxylation reaction followed by treatment with a mineral acid. However, this process also requires the use of an expensive alkaline metal formate and cannot solve the problems of the process disclosed in Patent document 2. Therefore, this process is not sufficiently advantageous.    [Patent document 1] Japanese Patent Application Laid Open No. Sho 49-11841    [Patent document 2] Japanese Patent Application Laid Open No. Hei 11-515031 (corresponding to WO 97/17315)    [Patent document 3] Japanese Patent Application Laid Open No. 2011-26232